This project involves the development and use of a new kind of microsopic technique, photoelectron microscopy, based on the photoelectric effect. A typical experiment consists of placing a specimen in a vacuum chamber and subjecting it to ultraviolet light. If the energy of the light source is sufficiently high, the sample surface can emit electrons (photoionize) as well as fluoresce. The photoelectrons are then accelerated, passed through a series of electron lenses and focused onto the phosphor screen (the image plane). This is an entirely different kind of experiment than conventional or scanning electron microscopy. However, photoelectron microscopy can be thought of as an extension of fluorescence microscopy using electrons instead of photons. The proposed new technique, photoelectron microscopy, is potentially a powerful tool for the study of biological surfaces, including the surfaces of cells in tissue culture. In the initial three-year period, the feasibility of this approach was demonstrated. A prototype instrument with magnification range x10- 200 was constructed and the first photoelectron images of organic and biological surfaces were reported. The depth resolution has been shown to be remarkably high: on the order of 15 A for the test compound, phthalocyanine. The current thrust is to measure the photoelectric properties of individual membrane components capable of higher magnifications.